1. Field of the Invention
The present invention relates to a method for manufacturing a disc and a method for manufacturing a stamper using an inorganic resist master.
2. Description of the Related Art
At the start of a full-scale HD (High Definition) video age due to popularization of digital broadcasting, increases in recording density of optical discs are advanced from DVD (Digital Versatile Disc) which is the mainstream at present to Blu-ray Disc (registered trade name).
In a mastering step of optical discs, patterns such as pits and grooves are formed by lithography using laser exposure. However, the recording density has been increased mainly by contracting exposure spots.
When a laser beam at wavelength λ is condensed by an objective lens having numerical aperture (NA) during mastering, the exposure spot diameter φ is 1.22×(λ/NA). Since objective lenses with NA of 0.90 to 0.95 close to the theoretical limit value 1 have been used from the beginning of development of CD (Compact Disc), shortening of the wavelengths of recording laser light sources have mostly contributed to contraction of exposure spot diameters.
Although He—Cd laser at a wavelength of 422 nm or Kr+laser at a wavelength of 413 nm has been used in mastering of CD, use of Ar+laser at UV (Ultraviolet) wavelength of 351 nm has permitted manufacture of DVD. Further, DUV (Deep Ultraviolet) laser at a wavelength of 257 to 256 nm has been put into practical application, and thus recordable Blu-ray Disc (BD-RE) has been realized.
According to an approach apart from this, there has recently been technology of realizing dramatically higher-density recording by a simple process, which has been introduced into manufacture of reproduction-only Blu-ray Disc (BD-ROM). Although organic materials (photoresist) have been used for photosensitive layers during lithography, there has been found development in which with a specified inorganic material, unexposed portions are dissolved by alkali development, and resolution is significantly improved as compared with an organic resist process.
Japanese Unexamined Patent Application Publication No. 2003-315988 discloses a technique in which an inorganic material is used as a photosensitive material. Inorganic materials having a resist function are referred to as “inorganic resist” hereinafter.
FIG. 11 shows protrusion/depression shapes after exposure and development in the case in which an organic resist is used as a photosensitive material and the case in which an inorganic resist is used as a photosensitive material.
In an organic resist process, recording is performed in a photon mode, and thus the minimum exposure pattern width is proportional to the exposure spot diameter and is substantially the same value as the spot diameter half-width value.
On the other hand, in an inorganic resist process, recording is performed in a heat mode, and thus when the threshold value of reaction temperature is sufficiently increased by design of a recording film structure, only a high-temperature portion near the center of an exposure spot contributes to recording, thereby permitting significant contraction of the effective recording spot diameter.
Therefore, pits of BD-ROM cannot be precisely formed by using an organic resist even at a DUV wavelength, but hen an inorganic resist is used, sufficient resolution is achieved  even by a blue semiconductor laser light source.
A semiconductor laser is capable of high-speed modulation on the GHz order and capable of precisely controlling a pit shape by introducing write strategy used for signal recording on phase-change discs and magneto-optical discs, and thus the semiconductor laser is suitable for achieving good signal characteristics. The write strategy is a method for recording one pit by high-speed multipulses. In this case, a pattern shape is optimized by controlling the pulse width, pulse strength, pulse interval, and the like of pulses.
The above-described inorganic resist process is described in brief.
As shown in FIG. 12A, an inorganic resist master 100 basically includes a layer structure in which a heat storage control layer 100b and an inorganic resist layer 100c are deposited in order by sputtering on a support (master substrate 100a) composed of, for example, a Si wafer or quartz.
In the inorganic resist master 100, as shown in FIG. 12B, a beam (recording light) modulated according to a record signal is condensed on the master surface through an objective lens with a NA of about 0.9 to perform thermal recording. The inorganic resist master 100 is installed on a turn table of an exposure apparatus and rotated at a speed corresponding to a recording linear speed to move relatively to the objective lens at a predetermine pitch (track pitch) in a radial direction.
After exposure is completed, the inorganic resist master is developed with an organic alkali developer such as tetramethylammonium halide (TMAH). As a result, protrusions/depressions corresponding to an exposure pattern are formed on the inorganic resist layer 100c. Namely, an exposed portion becomes a depressed portion corresponding to a pit shape or groove shape in the master.